Cathode ray tube (CRT) display devices have been rapidly replaced with liquid crystal display devices. A liquid crystal display device has features of energy saving, a flat body, and a lightweight body, for example. For this reason, the liquid crystal display device has been widely applied to a liquid crystal television, a monitor, a mobile phone, and the like. In order to further take advantage of such features of a liquid crystal display device, there may be proposed an improvement of an illumination device (what is called a backlight) which is provided on a backside of a liquid crystal display device.
A backlight is roughly classified into a side light type (also referred to as “edge light type”) and a direct type. A backlight of the side light type has an arrangement in which (i) a light guide plate is provided on a backside of a liquid crystal display panel, and (ii) a light source is provided at a side end part of the light guide plate. Light emitted from the light source is reflected in the light guide plate, and then is emitted from an upper surface of the light guide plate, so that a surface of the liquid crystal display panel is irradiated with the light uniformly. With the arrangement, it is possible to cause an illumination device (i) to have a thin body and (ii) to be excellent in luminance uniformity. For the reasons described above, the backlight of the side light type has been mainly used in a small and medium sized liquid crystal display devices, such as a mobile phone, and a laptop computer.
Patent Literature 1 describes an example of the backlight of the side light type. Patent Literature 1 describes a backlight having an arrangement in which (i) a buffer plate is provided between a light source and a light guide plate, and (ii) both the light guide plate and the buffer plate have a prism shape. This backlight takes advantage of (i) a refractive index of the light that is emitted from the light source and is emitted from the buffer plate and (ii) a refractive index of the light entering the light guide plate. With the arrangement, the backlight has an improvement in uniformity of light emitted from the light guide plate.